Electric power is produced by means of generators connected to the electric power transmission network. The transmission network transmits the produced electric power to consumers, such as electric motors, lighting equipment and the like. There is no economic way of storing electric power whereby the produced electric power must be consumed as it is produced. In other words, the amount of consumed energy must match that of produced energy.
Generators are electric motors designed for producing electricity, the rotors of which rotate at a certain speed. Usually the rotation speed of the generators is the same as the frequency of the electric power transmission network. Usually the frequency of the transmission network is 50 Hz. However, as the demand of the electric network fluctuates constantly, i.e. the network is in a dynamic state, the electric network must be able to accommodate a certain amount of imbalance between the produced electric energy and the consumed electric energy. This imbalance can be seen as small fluctuations of the network frequency. The rotating masses of generators and electric motors using electric power can compensate for small and short-duration imbalance situations, because energy is stored in the rotating masses in the form of inertia.
If the imbalance between the production of electricity and the demand of electricity is larger and/or its duration is longer, the network frequency tends to change more. This causes protection operations in the network, such as isolating a part of the network from the rest if the network or connecting reserve power to the network. The purpose of the protection operations is to keep as large a part of the electric power transmission network as possible in use during the fault. For example, if there is a sudden increase of demand in the network with a relatively long duration, the generators can not produce the necessary power as fast as needed and the rotating masses of the motors can not compensate for the increase of demand for a sufficiently long time. Thereby the electric network frequency starts to decrease. Due to the decrease of the frequency a part of t he network is isolated from the rest of the network until the generators can produce the necessary additional power subsequent to which the part of the network can again be connected to the rest of the network. A corresponding operation can take place in case a large generator of the network suddenly shuts down and other generators must replace the production of the non-operational generator. The above-mentioned relates to both the main network and dedicated electric networks, such as the electric network of a marine vessel.
Nowadays frequency converters are used in a number of applications. With a frequency converter the electric power supply can be dimensioned to exactly correspond with the demand of the electric motor or other electrical appliance. For example, a squirrel cage motor of an air blower does not have to be always driven at full speed, if partial rotation speed is sufficient for most of the operation time. By using a frequency converter it is possible provide partial rotation speed and to save a lot of energy.
In a normal frequency converter alternating voltage is converted to direct voltage which is then converted back to another alternating voltage. The second alternating voltage is used for directing electric power to a load, such as an electric motor. The second alternating voltage can also be controlled. The frequency controller drive, however, cuts the direct connection between the electric motor and the production of electricity, whereby the rotation inertia of the electric motor no longer can take part in compensating for smaller load variations in the network. However, the demand of the electric power of the electric motor tends to stay constant for a while regardless of the frequency and phase of the network.
FIG. 1 illustrates an example of an electric network 1 of a vessel having generators 2, 3, 4. Electric motors 7, 8 are connected to the network through motor-specific frequency converters 5, 6. The shafts of the electric motors are connected to the propellers 9, 10 of the vessel. The electric motors of a vessel are large, especially when their power is compared to the size of the electric network of the vessel.
The dedicated electric network of the vessel is relatively small. Due to this, the fault of even one of the generators of the vessel is a considerably serious fault situation and it can be seen as a change of the network frequency. For example, if one of the generators of the vessel becomes faulty, it will take a few seconds before the other generators can compensate for the power of the faulty generator. Usually, diesel-operated motors are used as generators of the vessel. The inertia of the large electric motors located behind the frequency converters 5, 6 can not take part in the maintenance of the network frequency. Thus the electric network 1 easily fails, i.e. protection operations quickly disconnect a part of the network or the whole network. If the network fails, the rotation speed of the propellers 9, 10 decreases. In propeller drive the relation between the rotation speed and the used power is immediate so that even a small change in rotation speed can be clearly seen in the power demand and vice versa. Thus, the maneuverability of a vessel soon decreases due to a fault in the electric network and the vessel stops. In case the duration of network partial power is long enough, the frequency of the network decreases sufficiently to trip the protection operations of the network. In the worst case, the entire network of the vessel must be shut down, whereby the propellers of the vessel no more provide propulsion power and the vessel becomes nearly impossible to steer. This can have catastrophic consequences.